From the HBD Archive
From: (Michael L. Hall)
Subject: Liquid properties
Date: 1992-07-02 16:46:45 GMT

Russ Gelinas writes:

> Hydrometer readings will go up even while the heavier trub material
>is settling out because the wort is cooling. Cool wort has a higher
>SG than hot wort. Think of it as a syrup; cold syrup is thick, hot
>syrup is thin, and in a simple sense, that's what SG is a measure of,
>liquid thickness.

I hate to nitpick, but SG is *not* like liquid thickness. Specific
Gravity is like liquid *density*. Viscosity is like liquid thickness.
For example, think about malt extract in a can: at room temperature,
it's very thick (viscous) so you heat it up in the can to make it
thinner (less viscous). Heating also has an effect on the density,
usually decreasing it. The malt extract may have a slightly different
density at the higher temperature, but it will have a very different

The difference between density and viscosity can be seen by comparing
mercury (aka liquid silver) and maple syrup. Which is the most dense
at room temperature? Mercury has an SG of about 13.5, where maple syrup
would be about 2 or 3 at the most, so mercury is by far the most dense.
Which is most viscous at room temperature? Mercury has a viscosity which
is similar to water, but maple syrup is much more viscous. This is one
case which shows that density and viscosity are definitely not the same

Mike Hall
Thermal Hydraulic Nut
and avid beer drinker

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